Intramolecular electron transfer (IET) in the oxidized dipeptide Tyr-Trp was investigated in the pH range from 1.0 to 3.1 by the method of time-resolved chemically induced dynamic nuclear polarization. The results were compared with data obtained earlier for Trp-Tyr. Surprisingly, it was found that the direction of JET changes with the order of the amino acid residues in the peptide. For Tyr-Trp, the rate constant of electron transfer from tyrosine residue to tryptophanyl cation radical is below 1.2 x 10(4) s(-1), whereas for Trp-Tyr, the value of this rate constant is 5.5 x 10(5) s(-1). Conversely, for oxidized Tyr-Trp at pH range 2.1 and lower, electron transfer from tryptophan residue to tyrosyl radical is observed. The rate constant of this reaction is proportional to the concentration of protons in aqueous solution, and at pH 1.0 is equal to 6.5 x 10(5) s(-1). The change in direction of JET observed for oxidized Tyr-Trp dipeptide is presumably due to the positive charge at the N-terminal amino group of the peptide, which promotes electron transfer in the direction of the N-terminus.